The present invention relates to the art of injection molding hollow plastic articles, especially hollow plastic articles which are subsequently reheated and blow molded into containers such as oriented hollow plastic containers. Polyethylene terephthalate (PET) is typical of the plastic material used.
The injection molded hollow plastic articles produced are preforms which are further processed. Naturally, a high rate of production is important in commercial operations, and the rate at which the injection molding cycle can produce preforms is limited by the time taken to cool the preforms sufficiently to allow post molding handling without damage to the preform.
The hollow plastic articles or preforms produced in the injection molding cycle generally have relatively thick walls, typically on the order of 0.060 to 0.160 inch, and are molded with PET resin at relatively high temperatures, typically of the order of 510.degree. F. to 580.degree. F. Consequently, after removal from the mold the hollow plastic articles must be cooled sufficiently to prevent their deformation or adhesion to one another. However, their thick wall acts as an insulator tending to trap heat inside the wall. Cooling the article in the mold by means of its contact with the cooled mold surfaces quickly chills the inner and outer skins, but mold cooling will remove the heat trapped within the wall only by keeping the article in the mold for an extended period of time. Naturally, this is economically prohibitive.
Initially cooling the molded articles skin is sufficient to permit ejection from the mold without damage, but additional cooling is then immediately required to remove additional heat as it is conducted to the skin. If the additional cooling were omitted, the skin temperature would rise and cause the molded articles to stick together, to become prone to surface damage, to bend, or warp, and or to crystallize. Naturally, this is unacceptable.
U.S. Pat. No. 4,527,970 to Murley and 4,586,891 to Ichizawa et al. cool the molded articles in the mold, followed by ejection and dropping onto a conveyor which transports the articles through an air cooled chamber. The mold cooling is sufficient to prevent the articles from sticking together as they drop onto the conveyor and the additional cooling provided downstream removes remaining heat in the thick walls. However, the machines are cumbersome and not entirely reliable. Also, the parts lose their orientation and must be unscrambled before downstream blowing operation.
Additional approaches to this problem are shown in U.S. Pat. No. 4,439,133 to Rees et al. and 4,449,913 to Krishnakumar et al. These patents show turret type injection molding systems wherein one of the turret stations in the operating cycle is used for the additional cooling. This approach will speed up the molding cycle, but it requires a dedicated machine with the resultant disadvantage of high cost. In addition, there is a reduced flexibility for producing different sized molded articles.
A horizontal injection molding system has been provided with a molded article transporter or robot which carries the articles from the mold in cooled tubes. U.S. Pat. No. 4,592,719 to Bellehache et al. shows air cooled tubes, and U.S. patent application Ser. No. 733,969 to Schad et al., filed May 14, 1985, and now U.S. Pat. No. 4,690,633, shows water cooled tubes which are tapered to maintain good surface contact with the molded articles as they shrink during cooling. This latter approach especially has effectively further reduced the time required for mold cooling and increased the production rate. However, the overall molding cycle has now been reduced to the point that there is insufficient time left for the molded articles to be completely cooled in the transporter or robot before it is needed to retrieve the molded articles from the next molding cycle. In order not to delay the molding cycle, the molded articles may be discharged from the transporter into another cooling device downstream. Naturally, this incurs additional cost, but also disadvantageously exposes the molded articles to increased risk of damage.
U.S. Pat. No. 4,209,290 to Rees et al. shows a vertical machine including an air cooled molded article transfer box to carry the partially cooled articles from the mold to the discharge station. The articles are hung vertically by their open ends which helps prevent bending of the articles during this cooling stage. However, this device has the capacity to handle the output from only one molding cycle thus limiting the amount of cooling that can be conveniently accomplished before the next molding cycle is completed.
It is therefore the principal object of the present invention to provide an apparatus for producing hollow plastic articles by injection molding enabling the attainment of a rapid operating cycle with extended cooling of the hollow articles.
It is a further object of the present invention to provide an apparatus as aforesaid which permits rapid injection molding cycles without the necessity for extended cooling time in the injection mold.
It is a still further object of the present invention to provide an apparatus as aforesaid using a transporter or carrier plate cooperating with an injection mold for extended cooling of hollow plastic articles formed therein accepting the articles from more than one injection molding cycle.
Further objects and advantages of the present invention will appear hereinafter.